So i'm going to replace my floorboards in my new-to-me 91 Comanche. but i haven't been able to find the reason WHY they rust in the first place.

why are they so prone to it and after i replace them is there a way to make sure the moisture stays out. i'm planning on line-xing the cab floor after the repair but is there a location that let's the water in typically or what?

Carpet and the felt pad underneath soak up moisture and hold it like a sponge, especially with a rubber mat over it. Old degraded door seals are a prime source of moisture entry but there's lots of other ways floor carpet gets wet.

__________________
***Chris***

"You can set my jeep on fire and roll it down a hill,
But I still wouldn't trade it for a Coupe DeVille."

cool..thanks guys. is there a better paint if i go that route and should i still coat the underside with a rubberized undercoat or something like that? i just didn't know if there was a culprit location for the moisture build up or something else that was letting water in for years.

cool..thanks guys. is there a better paint if i go that route and should i still coat the underside with a rubberized undercoat or something like that? i just didn't know if there was a culprit location for the moisture build up or something else that was letting water in for years.

help is appreciated! thanks.

Like chris said, the door seals are a big spot for water to get in. Especially the passenger side. Water in the glove box is a big indicator of that. You can replace them with 97+ XJ seals for a much better seal. Also make sure all your holes in the firewall are sealed well.

Government regulations on paint (mostly getting rid of lead paint) changed a lot around when early XJs and MJs were manufactured which is why most vehicles made in the late 80s and early 90s had crappy paint jobs that didn't last long. A quality undercoating will work just fine for you. Just make sure to use a good primer on both sides before lining or coating.

If you undercoat the bottom make sure to inspect & clean it thoroughly otherwise you may seal in some rust which would cause problems later . I found water soaked insulation/padding under the carpet that was still wet this past summer even after i pulled it out of the water 4 years or so ago so it sure likes to hold moisture.

Here is a picture of the project iam working on atm it sat in water for 15 something years . The unibody frame supports are still solid surprisingly so its just sheet metal work that needs to be done . I post it as you can see where some of the problematic areas are .

I have found rust holes in the pan area of where my wiper motor sits, even on the pass side where leaves and pine needles collected. Check the back window seals also.
Another thing I don't like is that the front bed drains are very close to draining right on top of the frame allowing added moister inside the frame, which the lowest part of the frame is right under where I see most floorboards starting to rust in that location. I am going to move the drain locations!
side vents - check the seals and also the wing window seals on the doors.
I am also making my floorboard upper frame supports "removable" so I can get in there for inspection every once and a while. Plus, if I ever have to do any future floorboard work, I do not want to remove 100's of spot welds! That's not very fun!